Add Python 3 support marker in setup.py

With the new markers, it's clear to PyPi users that xattr works both on Python 2 and 3.
This should be sufficient for xattr to show in green on the [Python 3 Wall of Superpowers](http://python3wos.appspot.com/).
Merge pull request #18 from phihag/setup.py-mark-py3-support

Add Python 3 support marker in setup.py